In fixed-bed fuels and lube hydroprocessing units, gas and liquid flow downward through multiple beds of solid catalyst. Heat is released from the catalytic reactions causing temperature to increase with distance down the bed. Cool hydrogen-rich gas is introduced between the beds to quench the temperature rise and replenish the hydrogen consumed by the reactions. Three requirements of an effective quench zone are transverse gas mixing, transverse liquid mixing, and quench gas mixing.
The introduction and mixing of quench into the process gas and liquid must be carried out in the interbed space which spans the full vessel diameter, but is often shorter than one vessel radius. Support beams, piping and other obstructions also occupy the interbed region so that unique hardware is required to perform efficient two-phase mixing in what amounts to limited volume.
Poor quench zone performance manifests itself in two ways. First, the quench zone fails to erase lateral temperature differences at the outlet of the preceding bed or, in the worst cases, amplifies them. An effective quench zone should be able to accept process fluids with 50 to 75 degree F. lateral temperature differences or higher and homogenize them sufficiently that differences do not exceed 5 degree F. at the following bed inlet. The second sign of poor performance is that inlet temperature differences following the quench zone increase as the rate of quench gas is raised. This indicates inadequate mixing of cooler gas with the hot process fluids.
Poor quench zone performance limits reactor operation in various ways. When interbed mixing is unable to erase temperature differences, these persist or grow as the process fluids move down the reactor. Hot spots in any bed lead to rapid deactivation of the catalyst in that region which shortens the total reactor cycle length. Product selectivities are typically poorer at high temperatures; hot regions can cause color, viscosity and other qualities to be off-specification. Also, if the temperature at any point exceeds a certain value (typically 800 to 850. degree F.), the exothermic reactions may become self-accelerating leading to a runaway which can damage the catalyst, the vessel, or downstream equipment. Cognizant of these hazards, refiners operating with poor internal hardware must sacrifice yield or throughput to avoid these temperature limitations. With present day refinery economics dictating that hydroprocessing units operate at feed rates far exceeding design, optimum quench zone design is a valuable low-cost debottleneck.
In U.S. Pat. No. 4,836,989 is described a method for quench zone design. The essential feature of this design is the rotational flow created in the mixing volume which increases fluid residence time and provides repeated contacting of liquid and gas from different sides of the reactor. This design is keyed to liquid mixing. More recent studies have shown it to be only a fair gas mixer. The trend to higher conversion and higher hydrogen circulation in fuels refining translates to gas/liquid ratios for which this design is not well suited. Height constrained units cannot be fitted with mixing chambers of the type described in this patent that are deep enough to effectively mix both the gas and liquid phases.
An interbed mixing system described in U.S. Pat. No. 5,462,719 offers some improvements over the design described above when gas mixing is paramount. This hardware is based again on a swirl chamber, but also includes at least three highly restrictive flow elements to enhance mixing, which necessarily increase pressure drop. Like the previously described system, this quench zone mixes the gas and liquid at once in a single chamber.
Another system, which is disclosed in U.S. Pat. No. 6,180,068, also provides enhanced mixing of quench gas and process fluids within the interbed space. This system employs separate mixing zones for each of two reactants permitting flexibility in mixing conditions while minimizing pressure drop as well as space and volume requirements. However, the efficiency of this device is sensitive to the degree of phase segregation achieved at the interbed inlet and thus may not perform as desired under all conditions and with respect to particular reactant characteristics.